For many years there has been a preference for nickel-containing austenitic stainless steel despite the high cost of the nickel content of these alloys. Alloys of the ferritic type, while potentially less expensive because of the complete, or nearly complete, absence of nickel therefrom, have the great disadvantage of being brittle, particularly after having bee welded and not subsequently annealed.
While there has been some progress in the improvement of the impact resistance of ferritic chromium alloys, thre has been no improvement in the ductility of ferritic chromium alloys in the as-welded condition until the inventions made by applicant and his co-workers, who filed the related applications referred to supra.
Applicant and his co-workers discovered that, if certain measures were taken to control the effects of carbon and nitrogen (C and N) in their ferritic chromium alloys, novel alloys could be made which remain ductile directly after welding, without requiring intervening annealing. These workers also discovered that their novel alloys possessed high resistance to intergranular attack, which is a serious problem in many chemical plant exposures. Moreover, the alloys also possess resistance to stress corrosion cracking, which can be important not only in chemical exposures but also as regards seawater. In addition, certain of the alloys to which molybdenum was added possessed resistance to pitting corrosion, which is another serious problem frequently encountered.
Now the present applicant has discovered yet another novel grouping of constituents by the use of which, in certain specific ranges, the properties of ductility in the as-welded condition, resistance to intergranular attack, general corrosion resistance commonly associated with the presence of molybdenum, resistance to stress corrosion cracking and other desirable properties can be attained, while yet eliminating certain undesirable properties of some of the earlier alloys and, at the same time, utilizing practical raw materials and recycled scrap under practical metallurgical operating conditions.
Application Ser. No. 371,951 discloses and claims an alloy using titanium and aluminum as additives to control and overcome the effects of the C+N contents of the alloys, and, for many uses, such alloys are highly suitable. However, for some applications, where highly polished surfaces are necessary, there may be some difficulty with surface specks resulting from the presence of titanium in the alloy. The invention of the present application cures this difficulty by substituting niobium for the titanium, thereby eliminating speck appearance on the surfaces. In addition, HNO.sub.3 attack on the weld metal of 26% Cr, 1% Mo + Ti alloy is high, but not if Nb is used instead of titanium.
British Patent No. 1,209,118 issued Oct. 21, 1970 to Seikosha (corresponding to Japanese patent application No. 42-36434 published 3/24/1972) discloses a ferritic chromium alloy for use in antimagnetic watch cases where high magnetic shielding together with corrosion resistance to ordinary atmospheric exposure accompanied by contact of the metal watch case with the human skin is essential. Although the Patent claims Cr from 16 to 25%, and Mo from 1 to 4%, the simple example reported was much more limited, consisting of Cr 19.2%, Mo 2%, C 0.03%, Nb 0.31%, with no reference made to aluminum or nitrogen contents, the only other materials specified besides the iron balance being silicon and manganese. In testing for corrosion resistance, the Patent states merely that "a very large number of different corrosion fluids" were used, one being a 10% hydrochloric acid solution in which the alloy of the Patent was reported to be merely discolored whereas 18-8 stainless steel, in comparison, exhibited slight corrosion.
U.S. Pat. No. 3,499,802 issued to Lagneborg discloses the use of niobium and aluminum among many combinations. In alloys containing 15 to 17.5% chromium, the aluminum is used to prevent the embrittlement of steel if held at 475.degree.C. and is used in the quantity of 0.5 to 4% by weight. It is stated that the niobium may be used to bind C+N as carbides and nitrides, the maximum niobium content being 1%. Molybdenum also may be used for the improvement of high temperature strength up to the quantity of 3 to 4% maximum. Although there are some examples given in the Patent containing more than 17.7% chromium, none of these examples show the inclusion of niobium or molybdenum. There is no reference to welding, to corrosion resistance after such welding, nor to the brittleness of welded materials.
U.S. Pat. No. 1,954,344 issued to Becket and Franks in 1934 is an early disclosure of the use of niobium in ferritic chromium alloys to "fix" the carbon present. There is no reference to the nitrogen content of the alloys. The amount of niobium specified to fix the carbon would, if the normal commercial quantities of nitrogen of alloys of the 1934 period were present, amount to perhaps about six times the combined carbon and nitrogen content. Only two of the examples reported in this Patent contain more than 20% chromium. These two have 22.5 and 22.85%, respectively. Only the latter had niobium added, this alloy containing 0.12% carbon but no specified nitrogen content. No aluminum was added and no references are made to the alloy properties if it were welded.
U.S. Pat. No. 2,183,715 issued to Russell Franks in 1939 discloses an improvement in the corrosion resistance, particularly pitting corrosion, through the inclusion of both niobium and molybdenum in ferritic chromium alloys. An example is given at 26% chromium, 0.10% carbon, 1.05% Cb (now called niobium, Nb), and 4.16% molybdenum, which, in the reported testing procedure in ferric chloride, showed freedom from pitting. No nitrogen content is given for this alloy but, if within the usual commercial range available at the time of the Patent, the niobium content would be perhaps only about 5 times the C+N content. The Patent specified, however, that nitrogen could be added, up to the extent of 0.5%, to improve the ductility and toughness, thereby teaching away from the findings of the present applicant. The Patent further states that aluminum addition would be permissible, but gives no examples and specifies no improvement in properties from such addition. There is no information concerning product properties after welding.
Franks' U.S. Pat. No. 2,190,790 (1940), in disclosing an oxidation resistant chromium steel, taught the desirability of increasing the nitrogen content of his alloys. Thus, Franks, too, teaches away from the findings of the present applicants.
German Patent Application No. 2,148,421, applied for on 9/28/71 and laid open to inspection on 4/27/72, convention date 10/23/70, describes the use of niobium as a carbon and nitrogen fixer in ferritic stainless steel containing 13-24% Cr, 1-3% Mo and niobium 24 times the carbon content. This steel is described as corrosion resistant and ductile after welding; however, there is a stated preference for heat treatment of as-welded articles after welding to restore plasticity. There are no examples of samples containing niobium, no reference to the use of aluminum or nitrogen, nor any limitations on either.
The present applicant, in application Ser. No. 153,259, filed June 15, 1971, disclosed ferritic alloys with chromium contents in the range of 19 to 35%, with titanium and aluminum together added to offset the effect of the C+N content, which latter are causative agents for brittleness and lack of resistance to intergranular attack after the alloys have been welded. Applicant has now done additional research and has found that, within a relatively restricted range of compositions of the several variables involved, it is practicable to substitute niobium for titanium.
Therefore, this invention relates to ferritic alloys containing niobium together with aluminum (and, optionally, molybdenum) which, within certain restricted ranges of C+N and chromium possess, in the as-welded condition, overall resistance to intergranular attack, stress corrosion cracking and exposure to various media promoting pitting attack as well as nitric acid, while simultaneously possessing ductility in the as-welded condition.